Zikmund M. et al. in Czech Patent No. 277548 B6 disclose preparation of hydrotalcite by reaction of (i) an aqueous solution of an aluminate of alkaline metal with (ii) solid magnesium compounds viz barrigtonite (MgCO3.2H2O), nesquehonite (MgCO3.3H2O), lansfordite (MgCO3.3.5H2O) hydromagnesite (Mg5(CO3)4(OH)24H2O) etc., for preparing an intermediate amorphous product which is kept mixed for 4 hrs prior to crystallisation for a period of 5-48 hrs at 50-120° C. Surface area (BET N2) was 29 m2/gm and 18 m2/gm for product crystallized at 115° C. for 8 hrs and at 100° C. for 14 hrs respectively. Average agglomerate particle size of product was <1.0 μm. Drawbacks associated with the process are longer aging time of 4 hrs to get an amorphous intermediate and longer crystallisation time of 14 hrs at 100° C. for product having surface area of (BET N2)<20 m2/gm. Further, the product is made using purer compounds of aluminium and magnesium and did not achieve the desired specification for lower particle size.
Reichle W. T. in an article “Catalytic reactions by thermally activated synthetic anionic clay minerals” in J. Catalysis 94, 547 (1985) discloses the preparation of hydrotalcite by standard aqueous precipitation and heat crystallisation procedure which imbibe the intercalation of solution of magnesium nitrate, aluminium nitrate, with solution of caustic soda and sodium carbonate. Interaction was completed within 4 hrs with vigorous agitation at temperature below 35° C. A product having surface area (BET N2) and particle size of <20 m2/gm and 1-2 μm respectively was obtained by heat crystallisation at 200° C. for 18 hrs. Drawbacks associated with the above process are longer addition and crystallization time as well as higher temperature under autogeneous pressure. Use of nitrate compounds of magnesium and aluminium is costly and at the same time creates disposal problem of ecologically unsafe waste containing nitrate anions.
Preston B. W. in U.S. Pat. No. 5,250,279 describes a process for production of synthetic hydrotalcite by reacting a mixture of magnesium hydroxide, a carbonate source, aluminium trihydrate of average particle size of 2.4 μm, or alkali metal aluminate, under hydrothermal condition at a temperature of 160-200° C. A product having purity of 99% and average particle size at ≦2 μm was obtained by using aluminate solution of low alkali content and molar ratio of magnesium source to bicarbonate in the range of 0.9-1.1. It is disclosed that at temperature below 160° C. the reaction proceeds very slowly and further that at temperature above 200° C. the product is of inferior quality. Drawbacks of the process are: (i) higher temperature of crystallisation (160-200° C.), (ii) critical control of molar ratio of reactant during synthesis. Low ratio of magnesium to bicarbonate results into product containing undesirable dawsonite as an impurity, (iii) sodium aluminate with ratio (Na2O/Al2O3) higher than 1.25, results in excess sodium difficult to wash. Moreover, higher ratio gives impurities of dawsonite in hydrotalcite, (iv) use of magnesium hydroxide, a source of magnesium, required longer filtration time, and high consumption of water to make it free from adhering electrolyte, and (v) stringent control of average particle size (<2 μm) of aluminium trihydrate.
Oza et al in U.S. Pat. No. 7,022,302 describe a process for preparation of synthetic hydrotalcite by reacting precursors of aluminium and magnesium at high temperature in presence of suitable alkali carbonate. The aluminium is taken from soluble aluminium salts such as non ferric aluminium sulphate. Further, there is no practical attempt of utilization of the effluent generated in the reaction. Even though the bittern originated from sea water is used as magnesium source, the other reactants used were of commercial grade. The process involved more unit operations to prepare magnesium carbonate and convert it into hydroxide by digesting in alkali.